am i the only one?

I am always comparing myself to my boyfriend's ex. I feel like we started dating too soon after their breakup and that i was some replacement or rebound. I keep stalking her to such extent and compare myself and it's starting to drive me crazy, i need to stop. Any little thing he does or says, i find someway to connect to her and wonder why he said it. I can't continue feeling this way. Any advice???